An anisotropic conductive film is used in one of processes for mounting electronic components whereby a semiconductor package is mounted on a printed wiring board, or conductor circuits formed on the surfaces of two printed wiring boards are electrically connected with each other and the two printed wiring boards are secured with respect to each other.
In the case of mounting a semiconductor package, for example, a semiconductor package having a connection section where a plurality of electrodes called bumps are disposed on a surface thereof which is to be placed on a printed wiring board for mounting thereon, and a printed wiring board having a connection section where a plurality of electrodes are disposed in the same pitch as the bumps are prepared. The semiconductor package and the printed wiring board are disposed so that the connection sections thereof face each other, with the corresponding electrodes on both connection sections being aligned to overlap one-on-one in the plane direction of the film, and are bonded together by thermal bonding with an anisotropic conductive film interposed therebetween, thereby mounting the semiconductor package on the printed wiring board.
In the case of connecting two printed wiring boards, two printed wiring boards each having a connection section where a plurality of electrodes are disposed in the same pitch are prepared. The two printed wiring boards are disposed so that both connection sections thereof face each other, with the corresponding electrodes on both connection sections being aligned to overlap one-on-one in the plane direction of the film, and are bonded together by thermal bonding with an anisotropic conductive film interposed therebetween, thereby connecting the conductor circuits on both sides and securing the two printed wiring boards with respect to each other.
The anisotropic conductive film used in mounting of electronic components typically has such a structure as a powdered conductive component is dispersed in a film containing a binder of various resins and has heat sensitive adhesion property. The content ratio of the conductive component in the anisotropic conductive film is controlled so as to have higher conductive resistance (referred to as “insulation resistance”) in the plane direction, in order to prevent short circuiting in the plane direction of the film, namely to prevent each pair of opposing electrodes facing each other with interposing the film therebetween from short circuiting with an other pair of adjacent electrodes within the surface.
When the anisotropic conductive film is used in thermal bonding, since the anisotropic conductive film is compressed in the thickness direction by heat and pressure applied thereto, content ratio of the conductive component in the thickness direction increases so that the electrically conductive powders are brought closer to or into contact with each other to form a network of electrical conductivity. As a result, conductive resistance (referred to as “connection resistance”) of the anisotropic conductive film in the thickness direction decreases. However, since the content ratio of the conductive component in the plane direction of the anisotropic conductive film does not increase, the initial state that the insulation resistance is high and electrical conductivity is low is maintained in the plane direction.
Thus the anisotropic conductive film has a property of anisotropic electrical conductivity, namely connection resistance is low in the thickness direction and insulation resistance is high in the plane direction. This property of anisotropic electrical conductivity enables the followings:
[A] while maintaining each pair of opposing electrodes independent from others by preventing the electrodes from short circuiting in the plane direction of the film;
[B] to establish good electrical conductive connection between each pair of opposing electrodes that face each other via the film. At the same time, it is also possible to secure a semiconductor package on a printed wiring board by thermal bonding or secure printed wiring boards with respect to each other by thermal bonding, by the heat sensitive adhesion property of the anisotropic conductive film itself. As a result, use of the anisotropic conductive film makes the operation simpler to mount electronic components.
Various metal powders have been put into practical use as the conductive component contained in the anisotropic conductive film, such as those consisting of powders of a shape such as granule, sphere, or lamella (scale, flake) having an average particle diameter ranging from several micrometers to several tens of micrometers. Particularly in recent years attention is drawn to a chain metal powder having a shape in which fine metal particles are bonded in a chain form.
Since the chain metal powder has large specific surface area than a granular ones, it has an excellent dispersibility to the binder. And it has lager aspect ratio, adjacent chain metal powders tend to connect with each other so as to easily form a network of good electrical conductivity while being dispersed in the film. Accordingly, the chain metal powder used as an conductive component makes it possible to form an anisotropic conductive film having better electrical conductivity in the thickness direction with smaller amount of filling than in the case of conventional powders.
Also in case the chain metal powder contains a ferromagnetic metal as described hereinafter, upon application of a magnetic field, the chain metal powder are oriented in a certain direction accordingly. For example, it is also made possible to further improve the anisotropic electrical conductivity of the anisotropic conductive film by applying a magnetic field in the process for the production of the anisotropic conductive film thereby orienting the chain metal powder in the thickness direction of the film. In order to have the chain metal powder oriented in the direction of film thickness, for example, such a process may be employed as to produce the anisotropic conductive film by applying a liquid mixture containing a chain metal powder and a binder onto a flat surface and solidifying the mixture by drying or other means, while applying a magnetic field to the mixture that has been spread over the flat surface and has not yet solidified, thereby solidifying the mixture in the state where the chain metal powder is oriented in the thickness direction so that the direction of orientation of the chain metal powder is fixed.
Use of the chain metal powder also makes it possible to produce an electrically conductive paste that enables to form an electrically conductive film having better electrical conductivity, an electrically conductive sheet having higher electrical conductivity or an active material compound for a battery having excellent collecting ability, while using a smaller amount of filling than in the case of conventional ones. Unprecedented applications may also be opened up by making use of the peculiar particle shape of the chain metal powder in such fields as capacitor, catalyst, electromagnetic shielding material, etc.
A chain metal powder containing a ferromagnetic metal such as Ni, Fe or Co, or an alloy thereof can be produced by the reduction deposition method, according to which, a lot of the fine metal particles are deposited by the action of a reducing agent in an aqueous solution containing ions of these metals. The submicron-sized fine metal particles made of the ferromagnetic metal or alloy in the early stage of deposition have a single magnetic domain structure or a similar structure, and are therefore simply polarized into bipolar state so as to exhibit magnetism. A lot of metal particles that exhibit magnetism are bonded in a chain form through the magnetism, thereby to form the chain metal powder. When the metal further deposits so as to cover the lot of metal particles that are bonded in the chain form, a chain metal powder is formed that the metal particles bond more firmly with each other.
However, the chain metal powder of the conventional reduction deposition method only produces a configuration such as a branching shape that many chains are branched out or, even when there are few branches, a bending shape that the chains are significantly bent or bent several times. The chain metal powders may be nonetheless useful, for example, in forming a good network of electrical conductivity in a binder. In order to make better use of the peculiar configuration of chain, however, it is preferable to produce a chain metal powder that has not only fewer branches but also has a linear shape or close to it. It is also important that the chain metal powder consisting of linear shape has small distribution of the chain length, in order to equalize properties when orienting a lot of chain metal powders in the same direction.
For example, the anisotropic conductive film is rendered the anisotropic electrical conductivity thereof by orienting the lot of chain metal powders in the thickness direction. With respect to the anisotropic conductive film having such a structure in order to reliably prevent short circuiting between adjacent electrodes which are arranged at very narrow pitch in the connection sections of the electronic component and the printed wiring board, it is required that:
[C] adjacent chain metal powders contained in the film do not form a network of electrical conductivity due to branching, namely the powders have as few branches as possible; and
[D] the chain metal powders oriented in the thickness direction do not cause short circuiting between adjacent electrodes even when the powders fall down in the plane direction of the film when a printed wiring board and an electronic component or two printed wiring boards are pressed so as to be bonded together with the anisotropic conductive film interposed therebetween, namely lengths of the powders are controlled to be less than the distance between the adjacent electrodes.
In order to meet the requirements described above, it has been proposed to carry out a reduction deposition method while applying a magnetic field to an aqueous solution. With this method, since a number of fine metal particles deposited in the aqueous solution can be bonded in a chain form while being oriented in the direction of magnetic field through the magnetism of the particles themselves, it is made possible to produce a chain metal powder that have fewer branches than in the case where magnetic field is not applied, and have linear shape.
For example, Non-Patent Document 1 describes that a chain metal powder consisting of linear shape can be obtained when Fe or Fe—Co is deposited while applying a magnetic field to an aqueous solution in a reduction deposition reaction conducted in the aqueous solution by using boron hydride as a reducing agent and that, in the case of Fe, it is necessary to apply a magnetic field of at least 10 mT, preferably 100 mT or more intensity in order to make the chain metal powder consisting of linear shape.
Non-Patent Document 2 describes that a chain metal powder can be obtained when Ni, Co or Fe is deposited in a reduction deposition reaction in an aqueous solution by using a trivalent Ti compound as a reducing agent, and that the chain metal powder consisting of linear shape of Ni can be obtained by applying a magnetic field of 100 mT during the reaction.
However, the chain metal powders produced by these processes include powders having some branches which can not be completely eliminated. Also since the above-described processes are not capable of controlling the chain length, the chain metal powder produced thereby is varying in length from very short to extremely long.
When the chain metal powder that have some branches and varies in length is used as a conductive component of the anisotropic conductive film, for example, the anisotropic conductive film may not have sufficiently high insulation resistance in the plane direction of the film even when the chain metal powder is oriented in the thickness direction of the film. Moreover, as the pitch between the adjacent electrodes becomes smaller, there increases a possibility that long particles of the chain metal powder to fall down in the plane direction of the film and cause short circuiting during pressure bonding.    Non-Patent Document 1: “Magnetic Properties of Single-Domain Iron and Iron-Cobalt Particles Prepared by Boronhydride Reduction”, A. L. Oppegard, F. J. Darnell and H. C. Miller, The Journal of Applied Physics, 32 (1961) 184s    Non-Patent Document 2: “Use of Ti(III) complexes To reduce Ni Co and Fe in Water Solutions”, V. V. Sviridov, G. P. Shevchenko, A. S. Susha and N. A. Diab, The Journal of Physical Chemistry, 100 (1996) 19632